1. Field of the Invention
This invention generally relates to distributing control information throughout optical networks. More specifically, the invention relates to modulating the optical signals transmitted within an optical network to encode control information in those optical signals.
2. Prior Art
There are many ways to modulate information onto an optical fiber communication system, including amplitude, phase, and frequency modulation. Recent advances in dense wavelength division multiplexing (DWDM) fiber optic networks have led to interest in developing the means to switch optical wavelengths (or lambdas) directly, rather than the conventional methods which require performing optical to electrical conversion, switching the electrical data, then converting back to the optical domain. Lambda switching technology transmits many high speed (2.5 to 10 Gbit/s or higher) data streams over a common fiber optic cable using different wavelengths of light; but unlike conventional DWDM, lambda switching also incorporates a set of emerging industry protocols to manage higher level functions such as switching and routing. These functions provide the ability to engineer traffic (specify how, when, and where data flows), ease network management, increase fault tolerance and reliability by providing redundant lambda paths, and implement new types of network topologies including ring mesh.
The key to lambda switching is the ability to automatically connect the endpoints in an optical network. Conventional networks require tedious, expensive configuration of each device, fiber, and protocol in the network; this approach is prone to errors which may leave critical data paths unprotected or exposed to potential single points of failure. Lambda switching automatically reconfigures the network by integrating the switching functions with higher level protocols; thus network designs can be constructed which would have previously been impossible to manage. There are many devices in a lambda switched network, including optical cross-connects (OXCs), DWDM devices, and others; the key to making all of these devices work together is the ability to share information about the network topology between all attached devices. This requires some form of inband communication path within the lambda switched network.
Conventional lambda switching proposals require a separate optical wavelength channel to be reserved for advertising network information to all attached devices. This information may include which devices on the network can be reached by which paths, available bandwidth, quality of service, and extensions of current internet routing protocols such as Open Shortest Path First (OSPF) to determine the optimal paths for data flowing through the network. Using this information, lambda switching devices can each construct a network topology map (or traffic engineering table) as a basis for subsequent wavelength switching operations. This is a self-constructed topology map, which changes over time without requiring input from the end users.
When a connection over the network is required, the ingress OXCs can transmit setup messages requesting that the downstream OXCs allocate one or more wavelength channels for the data. This signaling will likely be accomplished using emerging standards such as Multi-Protocol Label Switching (MPLS), or adaptations of the Resource Reservation Protocol (RSVP) currently employed on internet connections. To engineer the traffic flow, extended MPLS or RSVP messages must flow over the network, using paths set up by the traffic engineering tables. Alternately, traffic flow can be handled by routers that manage the network against some connection criteria, such as bandwidth utilization. Fault tolerance is achieved by requesting backup light paths, and services such as virtual private networks (VPNs) can be implemented. After processing the setup messages, the OXCs would signal successful light path resource allocation by passing MPLS or RSVP messages to upstream neighbor devices. The optical connection is completed and ready to transmit data from the edge of the network devices when each OXC between the two endpoints has assigned a label to the light paths.
The basis of wavelength switching is the ability of network devices to transmit their state and reachability information inband over the network. Based on this, wavelength routers build granual network topology tables automatically. It is then required that the wavelength routers send requests for primary and backup light paths to selected devices, which acknowledge when the light paths are labeled and ready for use. Thus, there is a great deal of control information passing around the network. One approach to realizing this model is to dedicate a separate control wavelength on the network to carry this information. This approach has some drawbacks; a failure in the single laser transmitting this wavelength for each device means that the device is no longer visible to the network fabric. Further, there are both technology and industry standards which limit the maximum number of wavelengths on the network to around 30–60, with each wavelength running at speeds of up to 10–40 Gbit/s or higher. It is not efficient to remove 40 Gbit of bandwidth from the network as overhead to implement the lambda switching control channel; this inefficiency will only increase over time, as channel modulation rates become higher.